A while loop in a while loop in ruby

Question

I'm relativly new to Ruby, or Programming at all, so please excuse me, if i'm making a "noob2 mistake with the logic here.

I am trying to get an output of rgba pixel values in binary for every pixel in an image (and it's location), using Chunky_PNG.

While the output is correct it only displays the first row, seems so as if the outer loop would only run once.

Is there a logical fault in it, or does a while loop in a while loop never work ? That it may be bad practice to do so, i can imagine, but i would still like to know why it doesn't what it's supposed.

require 'chunky_png'
image = ChunkyPNG::Image.from_file('test.png')

#vars
$width0 = 0
$widthmax = image.dimension.width.to_i
$height0 = 0
$heightmax = image.dimension.height.to_i

#main
while $height0 < $heightmax  do
    while $width0 < $widthmax do 
        puts image[$width0,$height0].to_s(2)[0..7] + " " + image[0,0].to_s(2)[8..15] + " " + image[0,0].to_s(2)[16..23] + " " + $height0.to_s + "," + $width0.to_s
        $width0 += 1
    end
    width0 = 0
    $height0 += 1
end

Why are you using globals (i.e. variables with a `$` prefix) for everything? — mu is too short, May 05 '13 at 20:03
Globals were te first type of variabels used in the tutorial i looked up, so i tought they were the first choice to use — x3nu, May 05 '13 at 20:34

score 2 · Accepted Answer · answered May 05 '13 at 20:02

You are missing a $

You have

width0 = 0

But you want

$width0 = 0

This has the effect of never resetting $width0 back to zero, so only the first row is output. It thinks the inner loop never has to run again, since $width0 is still at its maximum value in every iteration after the first one.

(I should probably also add that globals are not the best idea, as others have pointed out, but you did ask for the reason why the script only output the first row. :) )

Thanks. (a typo, shame on me ...) – x3nu May 05 '13 at 20:30 — x3nu, May 05 '13 at 20:30

score 0 · Answer 2 · answered May 05 '13 at 23:01

0

The error is the missing $ sign as explained by Ray Toal

Even easier it is to to use

each

for the loops. Then you don't need to handle the loop index yourself

($height0..$heightmax).each do |height|
  ($width0..$widthmax).each do |width|
    puts image[width,height].to_s(2)[0..7] + " " + image[0,0].to_s(2)[8..15] + " " + image[0,0].to_s(2)[16..23] + " " + height.to_s + "," + width.to_s
  end
end

answered May 05 '13 at 23:01

ovhaag

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your each variant isn't equivalent since you use inclusive ranges. Since we're indexing from 0, it's even more convenient to use `times`: `heightmax.times {|h| widthmax.times { |w| puts image[w,h]... } }` – dbenhur May 06 '13 at 00:41

A while loop in a while loop in ruby

2 Answers2